Do you mean ambient light during night-time? Twilight? Moonlight? You will need an spectroradiometer, that has an absolute spectral irradiance calibration. What do you use for measuring the spectral reflectance? If you are using an array spectrometer like Ocean Optics, Avantes, StellarNet or similar, it might be possible to measure spectral irradiance with it, if it is sensitive enough, you get a cosine diffuser for it, and get it calibrated. In the range 300-800 nm an array spectrometer should be good enough if it is sensitive enough, but you will need to check with a UV blocking filter that stray light near 300 nm is not a problem under your conditions. If you do not need fine spectral resolution then a wide slit will help increase the signal. If you need to measure extremely low levels of light, then you might need an array spectrometer with a cooled array, like those used for measuring fluorescence. (A cooled array, has lower electrical noise, so it allows the use of longer integration times.) You need to give us some idea of the irradiance level you want to measure... that would let us to give you a better answer.

Of course, if you will be measuring very low light levels, you will need to be doubly careful when measuring the spectra: you will need to avoid shading the sensor, as usual, but also to be careful to avoid that the laptop screen, or any other light source you use affects your measurements.

Do you mean ambient light during night-time? Twilight? Moonlight? You will need an spectroradiometer, that has an absolute spectral irradiance calibration. What do you use for measuring the spectral reflectance? If you are using an array spectrometer like Ocean Optics, Avantes, StellarNet or similar, it might be possible to measure spectral irradiance with it, if it is sensitive enough, you get a cosine diffuser for it, and get it calibrated. In the range 300-800 nm an array spectrometer should be good enough if it is sensitive enough, but you will need to check with a UV blocking filter that stray light near 300 nm is not a problem under your conditions. If you do not need fine spectral resolution then a wide slit will help increase the signal. If you need to measure extremely low levels of light, then you might need an array spectrometer with a cooled array, like those used for measuring fluorescence. (A cooled array, has lower electrical noise, so it allows the use of longer integration times.) You need to give us some idea of the irradiance level you want to measure... that would let us to give you a better answer.

Of course, if you will be measuring very low light levels, you will need to be doubly careful when measuring the spectra: you will need to avoid shading the sensor, as usual, but also to be careful to avoid that the laptop screen, or any other light source you use affects your measurements.

Thank you for that, some interesting leads for me to follow. More detail: I am using an OO USB2000 spectrometer, deuterium halogen light source and OOIBase32 software, to measure the complete reflectance spectra of terrestrial, nocturnal, salamanders. What I want to do is measure the spectra present in the natural ambient light when they are active (so twighlight, heavy shade, moonlight, etc), to fully assess what wavelengths are relevant to my study system before modelling and evolutionary/ecological inference. While everyone seems to agree this is important I can only find papers which either use diurnal species or simply ignore it.

However, even with just the above you have pointed me at the right tools to look at, so thank you.

James, this is an interesting issue, but there are more than one questions to be asked before you decide on a strategy. From the spectral measurement point of view I would separate the measurements in two steps (if you can do it with your OO USB2000): measure the spectral reflectance (SR) of the salamanders and measure the spectral power distribution (SPD) of the illumination under whatever conditions you are interested in. SR is a relative quantity, you measure the reflected light as compared to the reflectance of the ideal white, and if you multiply the result (wavelength by wavelength) with the SPD you get the total spectral radiance (TSR) for whatever illumination you measured the SPD - and the TSR is what determines what the observer shall see. Of course, to get the full picture you would have to calculate also with the spectral sensitivity of the observer: if we are talking about humans you would use the standard observer, which of course is only valid above a certain illumination level (i.e. in the photopic range). But, as you want to model evolutionary/ecological inferences you may want to know the spectral sensitivity of the natural enemies resp. preys of the salamanders, and this is where it gets really complicated.

I work with plants, so I can best answer about the light measurements. If you could find out which grating and slit are installed in your USB200, you may be able to guess whether the configuration is sensitive enough. However, to roughly test if your USB2000 would be usable under your setting you do not need to have it calibrated, you can base your decision on how long an integration time you will need to get a spectrum that has close to the maximum number of counts at its peak. I guess you are using a probe rather than an integrating sphere for measuring reflectance. If it is a probe (one with a 90 degree tip would best, or a normal fibre) you can use it without the light source to measure the "spectrum" as counts. It will not be irradiance, because the aceptance angle will be much narrower than that of a cosine diffuser and the shape of spectrum will not be true, as the sensitivity of the "pixels" in the array inside of the USB2000 depends on the wavelength. However, this test would give you a very approximate idea of whether your USB2000 is usable. A very rough rule-of--thumb would be not to exceed a very small number of seconds (e.g. 2000 ms). We do not have a USB2000, we have a Maya2000Pro, which has a different type of array detector, so I am not sure what would be a usable maximum integration time... but again it can be assessed by looking at the number of counts in dark scans (with the probe covered with a heavy black cloth, or something equivalent). Probably, it would be best to do the dark signal test with the "electrical dark noise correction" both enabled and disabled in the software... Again we are using SpectraSuite, and I do not know how different it is from your OOIBase32.

Thank you both. I have only just started the work and no one in my department has amy experience so your responses are greatly appreciated, though I feel it will take a while to get to grips with. Robert: I would ideally like to get to the point of analysing my data in terms of predator visual systems. I am a little bit away from that but am thinking about it, thank you for the pointers. Pedro: yes, I am using a probe at 90 degrees to my sample (~1mm from the surface using a tip). I had never heard of a cosine diffuser before but I think I will try and get my hands on one. OOIBase32 I believe was the software OO released before it's SpectraSuite series, currently we are trying to get our hands on a copy of the later but OOIBase32 seems to be fine.

SpectraSUite has also been replaced by another similar software with a different name. SpectraSuite is not very good. The user interface is counter-intuitive. If OOIBase32 allows you to do the measurements you need, then, there is probably no reason to replace it. I have recently co-edited a handbook on methods for UV photobiology research in plants. It is much easier to measure visible light than UV radiation, but the same general principles apply. It is open access so you may want to have a look, either through my list of publications here in ResearchGate or at http://uv4growth.wordpress.com/ (look for the "Handbook" tab and you will find the link to the .pdf of the latest "printing").